When polyester polymer particles are molded into articles, they are first dried to remove moisture that otherwise would cause chain scission and excessive loss of intrinsic viscosity (It.V.) during the molding process. The drying time can be shortened by increasing the drying temperature. To increase the drying temperature, the polyester particles must first be crystallized to prevent the pellets from sticking at their glass transition temperature (about 80° C.). By first crystallizing the polyester particles, drying temperatures of up to about 190° C. can be achieved, above which the heated air begins to cause the polymer to yellow.
Pellets are typically crystallized at about 140-180° C. for about 40 min. to 1 hour, then preheated to about 180-215° C. for 1-4 hours, and then solid state polymerized at about 200-220° C. for about 8 to 12 hours to increase their molecular weight in the solid state. The high temperatures and long times in the crystallization and solid stating processes combined typically impart to the pellets a degree of crystallinity in excess of 50% and melting points above about 220° C. Such high crystallinity and high melting point in the polymer is a disadvantage in the extrusion zone for a molding process since more energy is required to melt the polymer particles as the degree of crystallinity and/or the melting point increases. This amount of energy applied to the crystallized pellets will depend on the nature of the polymer since each polymer has a different latent heat of fusion, and the degree of crystallinity in any given pellet. For a given polymer composition, increasing the degree of crystallinity increases the mass of the crystalline regions, thereby increasing the energy required to melt the particles. Accordingly, it would be desirable to melt extrude particles with a lower degree of crystallinity to save energy costs. The energy cost savings can manifest itself in a number of different ways, including reduced total power consumption to the screw motor and heating elements. Alternatively, the same amount of energy can be applied to more efficiently process the pellets by reducing the time to melt, thereby reducing cycle time and/or the generation of acetaldehyde.
With the use of polyester polymer particles having high levels of crystallinity and/or high melting points, a higher temperature is frequently required to process the polymer resulting in an increase in the production rate of acetaldehyde, thereby increasing the overall amount of acetaldehyde present in the melt or articles formed from the melt. Therefore, the polyester polymer particles fed to the extruder or injection molding machine should also have a low level of residual acetaldehyde to help reduce the overall level of acetaldehyde in the melt and the articles formed from the molten polymer.
Prior to solid-state polymerizing polyester particles, pellets are also prepared in crystallizers at high temperatures in order to mitigate agglomeration in solid stating zones which operate at high temperatures needed to advance the molecular weight of the particles. This crystallization process combined with the annealing or perfecting of the crystals during the long hot solid-stating process imparts to the pellets high melting points of about 220° C. and more. Also, with high melting points, the temperature applied to the pellets in the dryer can be increased. However, the high melt points have the disadvantage of also requiring a slight increase of the temperature of the polymer melt in the extruder by a few degrees, thereby increasing the cooling time of molded products, increasing cycle time, and increasing the potential for more acetaldehyde formation. Moreover, in the solid stating process, the high temperatures applied to the polymer can anneal the polymer, with the effect that at times, some particles do not completely melt in the extrusion zone and thereby cause deformities in the molded product.